As a well-known type of steam turbine, one is known which is provided with a casing, a shaft body (rotor) installed inside the casing so as to be rotatable, a plurality of turbine vanes arranged by being fixed to an inner circumference portion of the casing and a plurality of turbine blades radially installed at the shaft body in the downstream side of the plurality of turbine vanes. Of these steam turbines, an impulse turbine converts pressure energy of steam (fluid) to velocity energy by turbine vanes and also converts the velocity energy to rotational energy (mechanical energy) by turbine blades. Further, in a reaction turbine, pressure energy is converted to velocity energy also inside turbine blades and the velocity energy is converted to rotational energy (mechanical energy) by reaction force derived from ejection of steam.
In the above-described types of steam turbines, normally, a clearance is formed in the radial direction between the tip portion of a turbine blade and a casing which surrounds the turbine blade to form a flow channel of steam. A clearance is also formed in the radial direction between the tip portion of a turbine vane and a shaft body. However, leakage steam passing to the downstream side through the clearance between the tip portion of the turbine blade and the casing does not impart torque to the turbine blade. Furthermore, in leakage steam which passes to the downstream side through the clearance between the tip portion of the turbine vane and the shaft body, pressure energy thereof is not converted to velocity energy by the turbine vane. Therefore, torque is hardly imparted to the turbine blade on the downstream side. Therefore, in order to improve the performance of a steam turbine, it is important to reduce the flow rate of the leakage steam (amount of leakage steam) which passes through the clearance.
As a related art, for example, Patent Document 1 proposes a structure in which a plurality of stepped parts are provided in the tip portion of the turbine blade in such a manner that the height thereof becomes gradually higher from the upstream side toward the downstream side in an axial direction; a plurality of seal fins extending toward each of the stepped parts are provided in the casing; and a small clearance is formed between each of the stepped parts and tip of each of the seal fins.
In the turbine, fluid which has flowed from the upstream side into the clearance collides with a step surface of a stepped part, thereby a main vortex is generated on the upstream side of the step surface and a separation vortex is generated on the downstream side (vicinity on the upstream side of the small clearance) of the step surface. Subsequently, the reduction of the leakage flow passing through the small clearance is achieved by the separation vortex generated in the vicinity on the upstream side of the small clearance. In other words, the reduction in the flow rate (amount of leakage steam) of the leakage fluid passing through a clearance between a tip portion of a turbine blade and a casing is achieved.